The proposed experiments are to determine the effects of acetylcholine and epinephrine on the automaticity of juxtanodal fibers, atrioventricular node, and His bundle of dogs hearts. The studies will: 1. measure the effects of acetylcholine on the automaticity of the AV node; 2. correlate the histology and transmembrane potentials of juxtanodal fibers; 3. measure the effects of acetylcholine and epinephrine on automaticity of juxtanodal fibers, and 4. determine the relative degree of automaticity of juxtanodal fibers, AV node and His bundle. The preparations consisting of juxtanodal fibers, AV node and His bundle are prepared by carefully removing the overlying endocardial and atrial tissues. The tissues are identified by their anatomical characteristics and further confirmed by their histological features. The preparations are placed in a tissue chamber and are superfused with Tyrode Solution equilibrated with 95% oxygen and 5% carbon dioxide. Microelectrode techniques will be used to record action potentials from fibers of the spontaneously beating tissues. Then acetylcholine or epinephrine will be added to the tissue bath to study its effects on the spontaneous activity of these fibers. The role of phase 4 depolarization, maximum diastolic potential, and threshold potential in the regulations of the spontaneous rate will be analyzed. In one study, the morphology and transmembrane potential characteristics of the juxtanodal fibers will be correlated. Results of the studies are expected to add original electrophysiological information and also better our understanding of certain types of arrhythmias.